If you have a key-wind and key-set watch, in most cases you do not have the original key for it and will most likely look for a newly made replacement key. But these keys can be problematic and often a sure way to mess up your watch. That makes it important for collectors to be careful and selective with the keys they are using.
The square shafts for winding (on the movement) and for setting (on top of the hands) are made of less hard steel (iron) than what is used for these new keys. This makes these square shafts very vulnerable for abrasion or damage.
Key and shaft often don’t match, due to different measurement systems for shafts and keys. Different keys with the same numbers vary in dimension from maker to maker. Moreover, these keys often do not even correspond to their own specifications, also with differences amongst themselves within the same key numbers.
Especially on shafts that are already showing wear and tear, it is important to have a size as close as possible to the shaft dimensions. On top of that (something most collectors don’t recognize as important): The inner square of these new keys (unlike old ones as shown in the first picture) does not go all the way to the front end. I don’t know why they came up with the idea to move that further inwards. This however means, that you do not have the full grip all along the shaft. The use of the wrong key can also lead to the shaft taking a conical shape sooner or later. This is also something you have to look for when selecting watches to buy.
As old keys are hard to come by, you more or less have to live with a set of new keys (better three or four to find the right size within the same key-number). You can also buy your keys separately, as you normally don’t use most numbers in the set. Once I found a suitable key, I leave it with the particular watch and replace it. I also look around for old keys (which all have the square inside going all the way to the end), but they are expensive and you have to get the right sizes, which is often a gambling game.
Whatever, there is one thing you should do: Take a Dremel or a regular file and take something off the front (when the square is ending further inside), to have the inner square going all the way up to the front. Even if you are not 100 percent in a 90 degree angle, this is in any case better than what you hade before. Make sure you are not leaving any abrasion in the key, which could get into your watch.
And you might be in for a few surprises: I had keys grinded/filed down, which showed a hole instead of a square further inside or a narrowing/widening. A perfect ‘tool’ to mess up your watch.